Ramsey Electronics FM25A Assembly And Instruction Manual page 18

After setting your DIP switches, recheck your 'math', depress the Program but-
ton and listen on a nearby FM radio for the carrier frequency. No audio input is
needed to make this first adjustment, you can simply listen for a "quieting" in the
normal background noise "hiss." Remember that if you have adjusted the
FM25A outside of 88 MHz to 108 MHz range it will default to 88.1 MHz. This
means that if you can't hear anything on the radio near the frequency you are
trying to transmit at, go to 88.1 MHz and see if it is there.
If you still can't receive anything then check the voltage at TP1. TP1 is located
at the lead of R10 that is closest to the outside of the board. With an operating
frequency of 107.9 MHz programmed into the DIP switches, there should be
around 4.5 volts at TP1. If there isn't, adjust L1 until you read 4.5 volts. With a
frequency of 88.1 MHz programmed into the DIP switches, you should read
about 0.5 volts at TP1. If you can't get either reading, see the troubleshooting
section.
2. Audio Connection
Adjust both Left and Right level potentiometers (R11, R16) to about half otf their
rotation. This is a good starting position. The best audio input for both testing
and general operation are the stereo "line-level outputs" of a cassette deck or
CD player. Most stereo systems have a variety of auxiliary output jacks of which
one or more are line-level outputs.
Hooking up an audio source to your FM25A is really quite simple. However,
there are some general rules:
•
A terribly distorted sound is a sign of too much audio level. Simply rotate the
level potentiometers, R11 and R16, CCW to reduce the level. Make sure
you rotate each one about the same amount to maintain proper stereo bal-
ance.
•
Stereo LP turntables are low-level output and will require the use of a pre-
amplifier for proper audio input to the FM25A.
•
NEVER connect the FM25A audio inputs to speaker outputs of a high
power stereo system; such a connection will destroy the IC chip.
•
Consider using a mike mixer for professional "radio station" sound. It will
allow you to easily fade, mix and switch between various audio sources.
3. Stereo balance
Adjust R6 for correct stereo balance. If you are transmitting in stereo adjust R6
to its center position. The better your ear for music and your understanding of
quality audio devices and interconnections, the better will be the performance of
your FM25A.
FM25A • 18